Steam and gas turbine oils are top-quality rust- and oxidation-inhibited oils. Steam turbines employ steam that enters the turbine at high temperature and pressure and expands across both rotating and fixed blades. Only the highest-quality lubricants are able to withstand the wet conditions, high temperatures and long periods of service associated with steam turbine operation. In gas turbines, they must withstand contact with very hot surfaces, often with intermittent operation and periods of nonuse. Therefore, to be effective, both types of oil must have, in addition to good corrosion protection and demulsibility, outstanding resistance to oxidation, which includes a minimum tendency to form deposits in critical areas of the system.
To achieve these desired properties, it is necessary to formulate these oils using a carefully balanced additive package. The nature of these fluids makes them very susceptible to contamination, particularly from other lubricants and additives. A relatively small degree of contamination can markedly affect the properties and expected service life of these lubricants. Further, to maintain effective operating conditions and to avoid damaging the equipment in which they are used, turbine oils should be kept meticulously clean and free of contaminants. Contamination is minimized by filtration of the turbine oils. To ensure that the turbine oils are substantially free of contaminants very fine filters are used.
The ratio between power output of turbines and oil volume has increased considerably over the years. This has resulted in a substantial increase in turbine operating temperatures. Therefore, it is necessary to protect the lubricant from oxidative degradation. The use of more antioxidants is one possible solution but higher treat levels sometimes lead to other problems such as sludge formation and solubility difficulties. A better approach is the use of synergistic antioxidant combinations, such as those taught in the present invention, that provide improved oxidation performance without causing sludge formation.
Due to the requirements of turbine oils, only a few classes of additives, relative to other types of lubricating compositions, are combined with the base oils. Generally, a finished turbine oil will contain only the base oil, antioxidants, rust inhibitors, demulsifiers, corrosion inhibitors and diluents, if necessary.
EP 0735128 A2 discloses extended life rust and oxidation oils comprising a dithiocarbamate and an alkylphenyl-.alpha.-naphthylamine. This reference does not teach the use of Group II or higher (i.e., Group III or Group IV) base oils, or the advantages obtained thereby, as required by the present invention.